A simple broadband anti-reflection (AR) coating consists of three layers. An AR coating reduces the reflectance of light with wavelengths within a spectral region. The first layer of the three-layer AR coating deposited on the glass substrate generally has a medium index of refraction, specifically higher than that of the substrate, and an optical thickness which is about one quarter of the reference wavelength of the spectral region. For visible light, this reference wavelength is typically in the range of about 500 nm to about 550 nm and frequently about 550 nm. The second layer has a high index of refraction, specifically higher than the first layer, and an optical thickness which is about one-half of the reference wavelength. The third layer has a low refractive index, specifically lower than the first layer and generally lower than that of the substrate, and an optical thickness which is one quarter of the reference wavelength. The three-layer design is described in Gaiser, U.S. Pat. No. 2,478,385; Thelen, U.S. Pat. No. 3,185,020; and Lockhart et al., xe2x80x9cThree-Layered Reflection Reducing Coatings,xe2x80x9d J.Opt.Soc.Am. 37, pp. 689-694 (1947). This three-layer AR coating is often referred to as the quarter-half-quarter (QHQ) design.
A disadvantage of the three-layer design is that the refractive indices of the three layers must have specific values in order to produce optimum performance. The selection and control of the refractive index of the first layer are particularly important. Deviation from specific refractive index values cannot be compensated for by varying the physical thickness of the layers.
A major improvement in the earlier AR coatings was introduced by Rock in U.S. Pat. No. 3,432,225. The Rock AR coating is made from two coating materials, one material having a high index of refraction, generally greater than two, and the other having a low index of refraction, generally lower than that of the substrate. The Rock AR coating consists of four layers. The first layer adjacent to the substrate is of the high-index material, and the second layer from the substrate is of the low-index material. The first and second layers have an effective optical thickness of about one-quarter of the reference wavelength. The third layer from the substrate is of the high-index material and has an optical thickness which is about one-half of the reference wavelength. The fourth layer from the substrate is of the low-index material and has an optical thickness which is about one-quarter of the reference wavelength. The advantage of the Rock AR coating is that materials with specific refractive index values are not required. The physical thickness of the layers can be adjusted to give a low reflectance value across the visible spectrum for a range of possible material indices.
However, economically producing an AR coating by sputtering is problematic. First, a preferred material for the high-index third layer, titanium oxide, has a slow deposition rate. The slow rate is only partially compensated for by running the sputtering tool used to deposit TiO2 at a much higher power. Thus, significant time and a number of sputter cathodes must be devoted to the sputtering of the thick third layer. Second, temperature-sensitive materials, such as plastic film which cannot take temperatures over about 60xc2x0 C., or cathode ray tubes (CRTs) which cannot be exposed to temperatures above approximately 150xc2x0 C., can be easily overheated by the high power of the TiO2 sputter sources. To avoid overheating, the TiO2 targets can be run at lower power, but only at the expense of a much slower coating process or one which requires many more TiO2 targets.
An optical coating and a method of coating a substrate are provided. A first layer is deposited adjacent to a substrate. The first layer has an optical thickness of about 0.27 xcex0 to about 0.31 xcex0. xcex0 is a reference wavelength corresponding to a spectral region bound by or in the visible spectrum. A second layer having an optical thickness of about 0.1 xcex0 to about 0.125 xcex0 and a refractive index from about 2.2 to about 2.6 is deposited. A third layer having a refractive index from about 1.46 to about 1.52 is deposited.
An advantage of the optical coating of the present invention is very low reflection for a broad range of wavelengths. The optical coating of the present invention has similar performance to the quarter-half-quarter (QHQ) design over a broad band of wavelengths.
Another advantage of the optical coating of the present invention is the thin high-index second layer. High-index materials typically have a low sputtering rate. The high-index second layer of the present invention is four to five times thinner than the high-index second layer in the QHQ design and the high-index third layer in the Rock AR coating. For example, for a reference wavelength of 550 nm, the physical thickness of the second layer may be from about 20 nm to about 35 nm. The thin second layer of the present invention facilitates high throughput production of the optical coating. Thus, the optical coating may be economically produced by sputtering.